In recent years, microwave plasma process apparatus has been attracting attention in the field of plasma process apparatus. The microwave plasma process apparatus has a plasma potential that is lower than that of other plasma process apparatuses such as parallel-plate-type plasma process apparatus or an ECR plasma process apparatus. Because of this, the microwave plasma process apparatus can generate plasma having relatively low electronic temperature and relatively low ion irradiation energy.
It follows that the microwave plasma process apparatus can prevent a substrate subjected to the plasma process from sustaining metal contamination or sustaining damage caused by ion irradiation, Moreover, since a plasma excitation space can be separated from a process space, a plasma process independent of a substrate material and patterns formed on the substrate can be accomplished.
The microwave plasma process apparatus has a slot antenna (electrode) for introducing microwave generated by a magnetron into a process chamber where the microwave may be 2.45 GHz, for example. The microwave generated by the magnetron is supplied to the slot antenna (electrode) through a waveguide and a coaxial pipe, and is introduced from the slot antenna into the process chamber.
Part of the microwave, when entering the slot antenna from the coaxial pipe, is reflected at a junction point, thereby returning to the waveguide. Moreover, when plasma is created directly under the slot antenna, the microwave will be reflected by the plasma to return to the waveguide through the coaxial pipe. In consideration of this, a matching device (tuner) is provided halfway through the waveguide so as to send back the returning microwave to the slot antenna. Between the magnetron and the matching device is provided an incidence monitor for detecting the incidence microwave. Control is engaged in such as to maintain a constant level of the incident wave detected by the incident monitor. This achieves such control that the microwave of a constant level is introduced into plasma-excited gas. Since the matching device is configured to send back the returning wave by calculating the load impedance in the process chamber, high precision may not be achieved to send back all the returning wave. In consideration of this, a filter is provided upstream relative to the incidence monitor to absorb the returning wave, thereby preventing the returning wave to reach the magnetron.
Such an incident and returning wave monitor is situated between the magnetron and the matching device, i.e., situated upstream relative to the matching device when the magnetron is regarded as an originating source. The incident monitor is thus able to monitor the reflected wave that is not sent back by the matching device. However, the electric power of the microwave reflected by plasma cannot be monitored.
The amount of a microwave reflected by plasma varies with the density of the generated plasma, so that the electric power of the reflected wave serves as an index indicative of plasma density. Since the conventional microwave plasma process apparatus cannot monitor the plasma-reflected microwave, however, it is not possible to control the introduction of a microwave based on the density of generated plasma.
Moreover, the frequency of a microwave generated by the magnetron has about a ±2% variation, and the density of generated plasma varies as the frequency of a microwave changes. That is, variation in microwave frequency is a factor that changes the density of plasma. The conventional microwave plasma process apparatus, however, does not monitor the frequency of a microwave that is actually supplied to the slot antenna. No measure is taken to prevent a change in plasma density caused by fluctuation of microwave frequency. Therefore, a change in the reflected wave caused by the variation of plasma density partly includes a change caused by the fluctuation of microwave frequency.